Who was the first to classify materials as "compounds"?

The concept of compound is probably as old as the concept of "element".
Robert Boyle, the key figure in the transition from alchemy to chemistry, was one was one of the first to try
to distinguish "compounds" from other types of matter. In the Sceptical Chymist (1661), Boyle refers
to chemical compounds as 'perfectly mixt' elements.
The term 'perfectly mixt' was meant to distinguish compounds from 'imperfect' mechanical mixtures. Boyle was
familiar with atomism and
believed that compounds involved chemical combination of atoms, an idea adopted by John
Dalton about 150 years later:

There are Clusters wherein the Particles stick not so close together, but that they may meet with Corpuscles
of another
Denomination, which are dispos'd to be more closely United with some of them, than they were among
themselves.[Boyle]

Boyle's "perfectly mixt bodies" classification did not distinguish true compounds from homogeneous mixtures
that were difficult to separate.

Antoine Lavoisier defined compounds in essentially the same way in his Traité Elémentaire de
Chimie (Elementary Treatise on Chemistry), published in 1789. Compounds were substances that could be
decomposed into elements; elements being "the last point which analysis is capable of reaching". His table of
33 elements included a few compounds which could not be separated with late 18th century technology (e. g.
chalk, alumina, and silica).

Definite composition is an essential characteristic of compounds included in the modern definition.
Many quantitative studies of metallic compounds like silver chloride in the 1700's lead to the recognition
that these compounds contained fixed, characteristic percentages of metal. French chemist Joseph Louis Proust
painstakingly determined the elemental composition of
a series of metal oxides, hydroxides, and sulfides. His results were remarkably consistent. In 1797, he
wrote:

We must recognize an invisible hand which holds the balance in the formation of compounds. A compound is a
substance which Nature assigns fixed ratios.

Proust's "invisible hand" was evident in many quantitative analyses and syntheses of compounds made by other
18th century chemists. The statement that when elements combine to form compounds, they do so in fixed
proportions by weight was soon referred to as "the law of definite proportions".

Proust's new law was immediately challenged by contemporary Claude Berthollet. Berthollet found that when
metals like copper or tin were heated in air, they formed oxides with variable composition. Proust performed
a series of analyses on Berthollet's copper oxides that proved that they were in fact mixtures of CuO with
Cu2O. He also showed that the "variable composition" tin oxide was a mixture of SnO and SnO2.

The law of definite proportions was later explained by John
Dalton. Dalton assumed that compounds are formed from combining atoms. If atoms of an element have a
characteristic weight, then the weight ratios of elements in compounds arise from the ratios of atoms
combined to make the compound. For example, if carbon monoxide is CO, and each carbon weighs 12 units and
each oxygen weighs 16 units, the weight ratio of carbon to oxygen should be 12:16. That implies that
converting 12 g of carbon into carbon monoxide will require exactly 16 g of oxygen, and will produce exactly
28 g of carbon monoxide. This is observed. Similarly, the fact that 12 g of C consumes 32 g of oxygen to
produce 44 g of carbon dioxide can be explained by proposing that there are 2 oxygens for every carbon atom
in this compound. The different carbon-to-oxygen ratios makes carbon dioxide and carbon monoxide two entirely
different compounds.

References, Bibliography, and Web Links

Robert Boyle's The Skeptical Chymist (1661) describes his classic experiments with gases and his speculations about elements and compounds, combustion, and calcination. Boyle's use of experimental results to bolster and develop his theories marks the emergence of chemistry as a science. CETI offers the text online; it has been reprinted by Kessinger Publishing, 1992. ISBN: 0922802904 http://www.library.upenn.edu/etext/science/boyle/chymist/ (01/11/99)